According to the growth chart in
an old thread, you'll get Masktchi when you neglect a female teen in an odd generation.
On my first generation on my Music Star, the same thing happened: I got a Masktchi from my Chamametchi. I was really surprised that I had taken "horrible" care of her when I looked at the growth chart.
But this is because there are a few factors when caring for Music Stars that you have to take into account.
To avoid getting worse-care characters, you should keep the hunger and happy hearts full, keep the weight low, keep the stress low, don't let it get sick, and play plenty of games to decrease the weight and stress.
So the reason you got Masktchi is probably because you missed on one or more things: hunger, happiness, weight, stress, or sickness.